This invention relates to new poly(biphenyl ether sulfones). More particularly, this invention relates to new poly(biphenyl ether sulfones) having improved color. This invention is also directed to an improved process for manufacturing poly(biphenyl ether sulfones).
Aryl ether polymers and copolymers are well known; they can be synthesized from a variety of starting materials and they can be made with different melting temperatures and molecular weights. Poly(aryl ethers) may be crystalline and, at sufficiently high molecular weights, they are tough, i.e., they exhibit high values ( greater than 50 foot-pounds per cubic inch) in the tensile impact test (ASTM D-1822). They have potential for a wide variety of uses, and their favorable properties class them with the best of the engineering polymers. Poly(aryl ether sulfone) polymers have become widely accepted for use under stress at high temperatures, often in excess of 150xc2x0 C.
One commercially important group of poly(aryl ether sulfones) comprises polymers containing a biphenyl group or moiety, typically derived from the monomer 4,4xe2x80x2-biphenol. Poly(aryl ether sulfones) that contain at least in part the 4,4xe2x80x2-biphenyl or 4,4xe2x80x2-biphenylene moiety are hereinafter referred to as poly(biphenyl ether sulfones).
Poly(aryl ether sulfones) having the following structure: 
are available from BP Amoco Polymers, Inc. under the tradename of Radel R(copyright). These resins possess excellent mechanical and other properties and are readily fabricated to provide a variety of useful articles such as molded goods, films, sheets and fibers. Poly(biphenyl ether sulfones) are also highly resistant to environmental stress cracking, and are thus particularly useful for manufacturing articles that are exposed to solvents or chemical agents at elevated temperatures and for extended times. For example, Radel R resins have found wide acceptance in the manufacture of articles for use where exposure to repeated and rigorous sterilization procedures is contemplated, such as medical trays and the like.
A very broad range of poly(aryl ether) polymers can be formed by the nucleophilic aromatic substitution (solution condensation polymerization) reaction of an activated aromatic dihalide and an aromatic diol in a substantially anhydrous dipolar aprotic solvent at elevated temperature. Ether bonds are formed via displacement of halogen by phenoxide anions with removal of halogen as alkali metal halide. Such polycondensations are usually performed in certain sulfoxide or sulfone solvents and the use of these dipolar aprotic solvents is an important feature of the process. The anhydrous dipolar aprotic solvents dissolve both the reactants and the polymers, and their use to enhance the rates of substitution reactions of this general type is well known.
One-step and two-step nucleophilic aromatic substitution processes for preparing poly(aryl ethers) are disclosed and well described in the art. In a one-step process, a double alkali metal salt of a dihydric phenol is reacted with a dihalobenzenoid compound in the presence of a dipolar aprotic solvent having a high boiling point such as, for example, dimethylformamide, N-methyl pyrolidinone, dimethyl sulfoxide, diphenyl sulfone or the like under substantially anhydrous conditions. In a two-step process, a dihydric phenol is first converted, in situ and in the presence of a solvent, to the alkali metal salt by reaction with an alkali metal or alkali metal compound. After removing water, a dihalobenzenoid compound is reacted with the double salt. The alkali metal salt of the dihydric phenol may be added in the solvent to the dihalobenzenoid compound either continuously, incrementally or all at once to achieve the polymerization reaction.
Several other variations of the process have been disclosed. An alkali metal carbonate may be employed with equimolar amounts of a dihydric phenol and a dihalobenzenoid compound at a ratio of at least one mole of an alkali metal carbonate per mole of dihydric phenol. The dihydric phenol reacts in situ with the alkali metal carbonate to form the alkali metal salt thereof, and the formed salt reacts with the dihalobenzeoid compound to form the polyaryl ether in the usual fashion.
Mixtures of sodium carbonate or bicarbonate and a second alkali metal carbonate or bicarbonate have been disclosed for use in the preparation of poly(aryl ether sulfones) and poly(aryl ether ketones), i.e. poly(aryl ethers) containing SO2 and/or CO linkages. The alkali metal of the second alkali metal carbonate or bicarbonate has a higher atomic number than that of sodium. The process provides polymer having a high molecular weight, as reflected by the reduced viscosity, that forms a tough, off-white film. Where fluorophenols or difluorobenzenoid compounds are used as the halogen-containing reactants, the amount of alkali metal carbonate required may be reduced.
Sodium and potassium salts, singly or in combination, are usually used in commercial practice. Although sodium salts are advantageous from an economic point of view, potassium salts are often chosen because the nucleophilic properties of the phenoxide anion are excellent. In a particular case where the dihalobenzenoid compound selected has low reactivity, a high molecular weight aromatic polyether cannot be obtained unless a potassium salt is used.
After completion of the polymerization reaction, additional process steps are needed to remove by-produced salts and to isolate and purify the resulting polymers. Recovery of dipolar aprotic solvents having high boiling points adds still further process steps.
Even though the monomers and solvents that are employed are highly purified, it is difficult to produce poly(arylether sulfones) that have low color, i.e. that are water white when formed and remain so when molded or otherwise melt processed. Side reactions, including solvent decomposition, hydrolysis of the dihalobenzenoid component and oxidation of a diphenol component or of phenolic endgroups, may occur during the heat-up portion of the process or later in the polymerization and lead to formation of highly colored contaminants. These, together with other contaminants produced by further thermal decomposition during subsequent melt fabrication operations, can result in products having an undesirable off-white, straw or even yellow color.
The poly(biphenyl ether sulfones) currently available to the trade, such as Radel R, have a yellow coloration. Although the effect on mechanical properties may be minimal, the cosmetic appearance of articles made from resins that are off-white or yellow may be unacceptable. Moreover, off-white resins are more difficult to pigment or color reproducibly to provide clear, bright colors such as are required by the packaging trade. Color, particularly of resins intended to be used in fabricating articles visible to the consumer, thus may be the determining factor in deciding the commercial acceptability of such goods.
Poly(biphenyl ether sulfones) having an improved, lighter color, preferably water-white, could find wider acceptance for many applications where color is a concern. Such lower color resins are clearly needed by the art and would thus represent a significant improvement over the resins currently available to the trade.
This invention is directed to an improved method for making low color poly(aryl ether sulfone) resins, and more particularly for making poly(biphenyl ether sulfone) resins, characterized by having a color factor of up to about 200, preferably up to about 170, determined on molded articles by spectrophotometric means. The improved process of this invention employs low particle size alkali metal carbonate, preferably anhydrous potassium carbonate, having an average particle size of less than about 100 microns, and may be conducted at a lower reaction temperature using reduced reaction times, compared with prior art processes.
The invention may be further described as directed to low color poly(biphenyl ether sulfones) characterized by having a color factor of up to about 200, preferably up to about 170, when molded.
The low color poly(biphenyl ether sulfones) prepared by the invented process have a superior appearance and are particularly desirable for use in applications where color, particularly a yellow color, is unacceptable such as, for example, in lenses, filters and other optical goods, for transparent covers or lids and in containers, glazing and other articles where transparency with low color is desirable or necessary. Lacking the yellow or beige cast of the prior art resins, the improved resins of this invention may also be more readily dyed or pigmented to achieve a desired coloration. The invented resins thus may also find use in filled and pigmented applications, particularly where white and brightly-colored goods are desired.
The invention has been described in related U.S. provisional application Ser. No. 60/186,864, filed Mar. 3, 2000, the entire disclosure of which is hereby incorporated herein by reference thereto.
The invented low color poly(biphenyl ether sulfones) comprise the repeating structural unit: 
wherein at least about 50, preferably at least about 75 mole percent and more preferably at least 90 mole percent of the divalent Ar groups is p-biphenylene (4,4xe2x80x2-biphenylene) having the structure: 
and wherein the remainder, if any, suitably comprises, but is not limited to, at least one member selected from p-phenylene, 4,4xe2x80x2-diphenyl sulfone and 2,2-diphenyl propane.
In general, polymers comprising high levels of the biphenyl or biphenylene moiety will have superior thermal properties and excellent chemical and environmental resistance. Thus, most preferred will be polymers wherein the Ar group is 100 mole percent p-biphenylene moiety, i.e. polymers wherein the aforesaid structural unit is: 
The poly(biphenyl ether sulfones) of this invention, when molded, will have a color factor of no more than 200, preferably no more than about 170, and will still more preferably lie in a range of from about 170 to about 80. Molded poly(biphenyl ether sulfones) with a color factor of from about 170 to about 100, preferably from about 150 to about 120, are readily produced by the improved process of this invention and these will be readily acceptable for use in most applications.
For the purposes of this invention, by color factor for the molded resin we mean a color value determined, preferably spectrophotometrically, using as a test specimen an article made from the resin such as, for example, an injection molded plaque, an extruded article or the like. The color factor may be conveniently calculated from the tristimulus values X, Y and Z that are obtained by integrating the transmission spectrophotometric curve.
Chromaticity coordinates x and y for a sample are determined as follows:   x  =                    X                  X          +          Y          +          Z                    ⁢              xe2x80x83            ⁢      y        =          Y              X        +        Y        +        Z            
Chromaticity coordinates define the as-measured color of the sample; color factor defines the color of the sample per unit of sample thickness. Color factor is determined by:       Color    ⁢          xe2x80x83        ⁢    factor    =                    (                              (                          x              +              y                        )                    -          0.6264                )            xc3x97      270        thickness  
wherein xe2x80x9cthicknessxe2x80x9d is the thickness of the specimen in inches. Where the specimen is a molded article having irregular shape, the thickness value will be the thickness of the portion of the test specimen presented to the spectrophotometer.
Color factor for the molded resin may also be measured if desired using other methods that will provide equivalent color factor values.
The poly(biphenyl ether sulfones) of this invention may also be characterized by the color of the as-produced resin wherein the transmission chromaticity values for a solution of the resin in a specified solvent at a standardized concentration are determined spectrophotometrically using a cuvette of known thickness. Measured at a concentration of 8 wt % in a solvent mixture of monochlorobenzene, sulfolane and N-methylpyrolidone, at a 3:2:2 ratio by weight, the invented poly(biphenyl ether sulfone) resins will have a solution color factor as-produced of up to about 50, preferably up to about 40. More particularly, the solution color factor will lie in the range of from about 10 to about 50, preferably from about 5 to about 40. Poly(biphenyl ether sulfones) having a solution color factor in the range of from about 20 to about 40 will also be highly desirable and acceptable for most applications. The solution color factor or batch color may be conveniently employed for quality control purposes in the production of poly(biphenyl ether sulfones).
The poly(biphenyl ether sulfones) of this invention may be prepared by the carbonate method. Generally described, the process is conducted by contacting substantially equimolar amounts of an aromatic bishydroxy monomer, preferably 4,4xe2x80x2-biphenol and at least one dihalodiarylsulfone, e.g., 4,4xe2x80x2-dichlorodiphenyl sulfone, 4,4xe2x80x2-difluorodiphenyl sulfone or the like, with from about 0.5 to about 1.1 mole, preferably from about 1.01 to about 1.1 mole, more preferably from about 1.05 to about 1.1 mole of an alkali metal carbonate, preferably potassium carbonate, per mole of hydroxyl group. Bishydroxybiphenyl analogs of biphenol, for example, compounds having structures such as 
and the like may also be found suitable for use as the aromatic bishydroxy monomer component or as a comonomer with 4,4xe2x80x2-biphenol. Copolymers comprising up to 50 mole %, preferably no more than 25 mole %, still more preferably no more than 10 mole %, of one or more other bishydroxy aromatic compounds such as 4,4xe2x80x2-dihydroxydiphenyl sulfone, hydroquinone, bisphenol A or the like may be prepared if desired by replacing an equivalent portion of the biphenol component of the reaction mixture with the selected comonomer.
The components are dissolved or dispersed in a solvent mixture comprising a polar aprotic solvent together with a solvent which forms an azeotrope with water, whereby water formed as a byproduct during the polymerization may be removed by azeotropic distillation continuously throughout the polymerization.
The polar aprotic solvents employed are those generally known in the art and widely used for the manufacture of poly(aryl ether sulfones). For example, the sulfur-containing solvents known and generically described in the art as dialkyl sulfoxides and dialkylsulfones wherein the alkyl groups may contain from 1 to 8 carbon atoms, including cyclic alkylidene analogs thereof, are disclosed in the art for use in the manufacture of poly(aryl ether sulfones). Specifically, among the sulfur-containing solvents that may be suitable for the purposes of this invention are dimethylsulfoxide, dimethylsulfone, diphenylsulfone, diethylsulfoxide, diethylsulfone, diisopropylsulfone, tetrahydrothiophene-1,1-dioxide (commonly called tetramethylene sulfone or sulfolane) and tetrahydrothiophene-1-monoxide. Nitrogen-containing polar aprotic solvents, including dimethylacetamide, dimethylformamide and N-methyl-pyrrolidinone pyrrolidinone and the like have been disclosed in the art for use in these processes, and may also be found useful in the practice of this invention.
The solvent that forms an azeotrope with water will necessarily be selected to be inert with respect to the monomer components and polar aprotic solvent. Those disclosed and described in the art as suitable for use in such polymerization processes include aromatic hydrocarbons such as benzene, toluene, xylene, ethylbenzene, chlorobenzene and the like.
The azeotrope-forming solvent and polar aprotic solvent are typically employed in a weight ratio of from about 1:10 to about 1:1, preferably from about 1:5 to about 1:1.
Generally, after an initial heatup period, the temperature of the reaction mixture will be maintained in a range of from about 190xc2x0 C. to about 250xc2x0 C., preferably from about 200xc2x0 to about 230xc2x0 C., still more preferably from about 200xc2x0 to about 225xc2x0 C. for about 0.5 to 3 hours. Typically, if the reaction is conducted at atmospheric pressure, the boiling temperature of the solvent selected usually limits the temperature of the reaction.
The reaction may be conveniently carried out in an inert atmosphere, e.g., nitrogen, at atmospheric pressure, although higher or lower pressures may also be used.
It is essential that the reaction medium be maintained substantially anhydrous during the polycondensation. While amounts of water up to about one percent, preferably no more than 0.5 percent by weight, can be tolerated, and are somewhat beneficial when employed with fluorinated dihalobenzenoid compounds, amounts of water substantially greater than this are desirably avoided as the reaction of water with the halo compound leads to formation of phenolic species and low molecular weight products are obtained. Substantially anhydrous conditions may be conveniently maintained during the polymerization by removing water continuously from the reaction mass with the azeotrope-forming solvent as an azeotrope. In the preferred procedure, substantially all of the azeotrope-forming solvent, for example, chlorobenzene, will be removed by distillation as an azeotrope with the water formed in the reaction, leaving a solution comprising the poly(biphenyl ether sulfone) product dissolved in the polar aprotic solvent.
After the desired molecular weight has been attained, the polymer will preferably be endcapped to improve melt and oxidative stability. Generally, the endcapping is accomplished by adding a reactive aromatic halide or an aliphatic halide such as methyl chloride, benzyl chloride or the like to the polymerization mixture, converting any terminal hydroxyl groups into ether groups.
The poly(biphenyl ether sulfone) is subsequently recovered by methods well known and widely employed in the art such as, for example, coagulation, solvent evaporation and the like.
In the improved process of this invention, low color poly(biphenyl ether sulfones) are obtained by employing solid particulate alkali metal carbonate, preferably anhydrous potassium carbonate, having a fine particle size. Preferably, the average particle size of the alkali metal carbonate will be no more than about 100 microns, preferably no more than about 80 microns, and more preferably no more than about 60 microns. Alkali metal carbonate, particularly potassium carbonate, with an average particle size of no more than about 30 microns may be found to be highly effective in producing low color poly(biphenyl ether sulfones). The alkali metal carbonate employed in the practice of the improved process according to the invention may be further described as a particulate potassium carbonate having an average particle size lying in the range of from about 10 to about 100, preferably from about 10 to about 80, more preferably from about 10 to about 60 microns. Particulate potassium carbonate having an average particle size of from about 10 to about 30 may be found particularly effective in producing the low color poly(biphenyl ether sulfones). By average particle size we mean the weight average particle size; for the purposes of this invention, the average particle size of the alkali metal carbonate will be taken to be the equivalent of volume particle size, which may be conveniently determined using a particle analyzer analytical instrument.
The use of such low particle size carbonate provides poly(biphenyl ether sulfone) having substantially improved color as reflected by a lower color factor. The use of low particle size carbonate also provides a given molecular weight polymer using shorter overall polymerization reaction times and allows the use of lower reaction temperatures, together affording a significant improvement in energy consumption, reducing production costs.
The invention will be better understood by way of consideration of the following illustrative examples and comparison examples, which are provided by way of illustration and not in limitation thereof. In the examples, all parts and percentages are by weight unless otherwise specified.